ooh, nasty - I had a blocked duct when my milk came in and it hurt like hell. Here's what I did and it cleared within 48 hours:
Ibuprofen 800mg X2 daily (relieves inflammation)
hot compress and loads of massage before feeding - gets some of the foremilk out and makes it easier for the baby to extract the rest (I'm assuming you're a bit engorged?)
cold compress for 20-25mins after feeding.
hope you're feeling better soon